Communications systems exist which utilize a modulated carrier signal having a nominal frequency in the high microwave to millimeter wave frequency band. This frequency range, extending from 18 to 50 gigahertz (GHz), is used or allocated for a variety of systems, including point-to-point and point-to multipoint private radio systems. In such systems, it is generally necessary to provide for electronic adjustment and/or continuous modulation of the oscillator output frequency.
While several different types of oscillator circuit designs are known for generating the requisite modulated carrier signals in the frequency range from 18-50 GHz, they all have limitations which render tham unusable for certain telecommunications applications. For example, one well-known type of oscillator circuit design for this frequency band utilizes Gunn diodes. The problem with these semiconductor devices is that they are not efficient and produce limited output signal power. Another oscillator circuit design uses Impatt diodes. These devices, like Gunn diodes, are inefficient, often require elaborate heat sinking and do not possess the reliability required to meet the objectives of many system applications. Still another oscillator circuit design for different portions of this frequency band utilizes transistor devices, including bipolar transistors and the various varieties of field-effect transistors (FETS), such as insulated-gate FETS (IGFETS) and metal on silicon FETS (MOSFETS). While this class of oscillator circuit designs provides improved efficiency and reliability, the output signal power is limited due to the low gain of these transistor devices in the frequency band above 18 GHz. Prior art attempts to increase the output power have focussed on increasing the area of the transistor device or cascading several such devices in series. Increasing the area of the transistor lowers its impedance and requires matching low impedance levels in the interfacing circuitry. Providing this matching low impedance in the interfacing circuitry produces high circuit losses which substantially negate the effect of increasing the output power. Moreover, cascading two or more transistors in series along with a feedback loop results in a design wherein the oscillator frequency stability oftentimes does not meet system performance objectives. Accordingly, a frequency stable oscillator circuit capable of providing higher output power in the high microwave to millimeter wave frequency band and whose nominal frequency is electronically adjustable would be extremely desirable.